linux-kernel - Re: [PATCH] kmsg: limit message size

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Message-Id: <20120508155726.d02d6600.akpm@linux-foundation.org>
Date:	Tue, 8 May 2012 15:57:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	arnd@...db.de, gregkh@...uxfoundation.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kmsg: limit message size

On Sat,  5 May 2012 16:56:12 +0200
Sasha Levin <levinsasha928@...il.com> wrote:

> There are no size checks in kmsg_write(), and we try allocating enough
> memory to store everything userspace gave us, which may be too much for
> kmalloc to allocate.
> 
> Furthermore, we can have an integer overflow if len==INT_MAX, in that case
> we'll corrupt kernel memory.
> 
> This was tested with several userspace programs that write to kmsg, and haven't
> found a case where the program attempts to write more than PAGE_SIZE.
> 
> Signed-off-by: Sasha Levin <levinsasha928@...il.com>
> ---
>  drivers/char/mem.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index d6e9d08..c90964b 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -815,6 +815,9 @@ static ssize_t kmsg_writev(struct kiocb *iocb, const struct iovec *iv,
>  	ssize_t ret = -EFAULT;
>  	size_t len = iov_length(iv, count);
>  
> +	if (len > PAGE_SIZE)
> +		return -E2BIG;
> +
>  	line = kmalloc(len + 1, GFP_KERNEL);
>  	if (line == NULL)
>  		return -ENOMEM;

Well, this is a write(), and write() is permitted to return
less-than-asked-for.  So what we could do here is to write the first N
bytes and then return N to userspace.  Well-behaved userspace will
notice this and then do some more writing from offset N.

Could I ask that you try this and test it a bit, see whether there is
any well-behaved userspace out there?  Things like cat and echo
_should_ dtrt.

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